1. Field of the Invention
The invention relates to a process for producing a shaped article from a contact material based on silver. The invention furthermore relates to a contact material of that type and to a shaped article or contact piece made of such a contact material.
Contact materials based on silver, which contain particular active components, have long been found useful for a contact piece in power engineering low-voltage switching equipment, for example in a circuit-breaker or in a contactor relay. Both metals and metal oxides are known as active components which favorably affect the switching properties of the contact material. Examples of metallic contact materials based on silver are silver-nickel (AgNi) and silver-iron (AgFe). An example of oxide contact materials which may be mentioned is silver iron oxide (AgFe.sub.2 O.sub.3).
Although oxide contact materials are less inclined to welding than metallic contact materials, the latter have a greater lifetime, in particular at low currents. In order to measure particular properties of a contact material, use is conventionally made of a test switch into which a contact piece made of the contact material is respectively fitted. Such a test switch is described in an article in Z.f. Werkstofftechnik/J. of Materials Technology 7, (1976), pages 381 to 389.
In particular, silver-nickel contact materials have good switching properties. However, there is the disadvantage that nickel dust which is formed during production or during operation by abrasion, and nickel oxide which is formed as a switching product, may have a detrimental effect on the human body.
European Patent 0 586 411 B1, corresponding to U.S. Pat. No. 5,422,065, discloses a contact material, based on silver, which contains the metals iron and rhenium as active components in proportions by mass of between 1% and 50% and between 0.01% and 5%, respectively. In that regard it has been found that rhenium improves the properties of a contact material of that type even in proportions by mass of less than 1%. That contact material is distinguished by a low degree of contact heating with a stable thermal behavior, a tolerable susceptibility to welding and a long lifetime in regard to predetermined switching current densities.
According to European Patent 0 586 411 B1, corresponding to U.S. Pat. No. 5,422,065, the silver-iron-rhenium contact material is produced by mixing silver and iron-rhenium alloy powder or by mixing separate silver, iron and rhenium powders. The powder mixture is then processed using molding presses or extruders and sintering to form shaped articles or to form semi-finished products. The structure of the material of that type, that is to say the size and the distribution of the active components in the silver matrix, is predetermined by the particle size of the commercially available metal powders or alloy powders. Using coarse metal powders leads to a coarse-grained structure, and using fine-grained metal powders leads to a fine-grained structure. The finest iron powders produced in industrial quantities have a mean particle size of about 5 .mu.m. Iron-rhenium powders are produced by atomizing a corresponding melt and likewise have a mean particle size of about 5 .mu.m or more.
There are essentially two known different powder metallurgy processes for producing a shaped article from the contact material. In the molding technique, the powder mixture is compressed in molding presses to form a molded part which is processed by sintering and, where appropriate, further pressing to form a finished shaped article. In order to produce a shaped article in the form of a contact piece, the molded part may additionally be pressed with a layer of pure silver for reliable connection of the contact piece to the support by silver soldering.
In a shaped article produced according to the molding technique, the active components added in powder form with a uniform particle size are distributed irregularly through the silver matrix. The structure of the shaped article is substantially isotronic.
In the extrusion technique, the powder mixture is firstly pressed and/or sintered to form a porous pressed part or billet. The pressed part or billet is extruded, where appropriate with a layer of pure silver (see above) to form a rod from which the shaped articles are detached and optionally subjected to subsequent treatment.
In a shaped article produced according to the extrusion technique, the powder grains of the silver and, where appropriate, of the active components, are deformed or aligned in the extrusion direction, as a result of which an anisotropic or, rather, linear structure is formed.
The electrical switching properties of the contact material, for example erosion, contact resistance and welding force, are substantially determined by its structure.
International Publication Number WO 95/08833, corresponding to U.S. Pat. No. 5,628,448, describes a method for connecting a contact piece made of a silver-metal oxide material to a holder by silver soldering or welding. In that case, the metal oxide of the contact material is at least in part reduced to metal in the surface region of the contact piece. No resulting structural change which favorably affects the switching properties of the contact piece or of the contact material is achieved. The described method is suitable merely for producing thin, weldable and solderable layers to a few hundred .mu.m.